Question

Впишите ответ, срочно!1!

Answer 1

$\left \{ {{b_{2}=6 } \atop {b_{4}=9 }} \right.\\\\-\left \{ {{b_{1}+d=6 } \atop {b_{1}+3d=9 }} \right.\\ -----\\2d=3\\\\d=1,5\\\\b_{1}=6-1,5=4,5\\\\b_{15}=b_{1}+14d=4,5+14*1,5=4,5+21=25,5\\\\\boxed{b_{15}=25,5}$